Plasma variable reactance device phase shifter



April 15, 1969 Filed Oct. 24, 1966 B. J. FORMAN ET AL 3,439,297

PLASMA VARIABLE REACTANCE DEVICE PHASE SHIFTER Sheet 4 ore April 15, 1969 a. J. FORMAN ETAL 3,439,297 .PLASNA VARIABLE REACTANCE DEVICE PHASE SHIFTER Filed-Oct. 24, 1966 Sheet 5 or 6 p 1969 B. J. FORMAN ETAL 3, 9,2

y PLASMA VARIABLE REACTANCE DEVICE PHASE SHIFTER Filed Oct. 24, 1966 Sheet 6 0r s 7- g 4mm? I 3m r" 161:: a; F P4 44; Al/Ir l" 1 l a Z0 0 2a 0 2a 0 Z0 p14 Pt? A23 1W United States Patent 3,439,297 PLASMA VARIABLE REACTANCE DEVICE PHASE SHIFTER Barry J. Forman, West Los Angeles, Ronald C. Knechtli,

Woodland Hills, and Yasuo Wada, Canoga Park, Califi,

assignors to Hughes Aircraft Company, Culver City,

Calif., a corporation of Delaware Filed Oct. 24, 1966, Ser. No. 589,120 Int. Cl. H0lj, 19/80, 7/46; H01p 1/18 US. Cl. 315-39 13 Claims ABSTRACT OF THE DISCLOSURE A microwave phase shifter comprising a waveguide for RF energy and a plasma variable reactance device for presenting an electronically controllable variable reactance plasma column therein. In the preferred embodiments the phase shifter is of the reflection type, the plasma column exists as a lumped reactance, and a suflicient number of plasma variable rectance devices are used to provide 360 degrees of continuous phase shift.

This invention relates generally to microwave phase shifters and in a preferred embodiment thereof to an electronically controlled microwave phase shifter of the reflection type employing a plasma varactor as the variable reactance element.

There are two general classes of phase shifters, i.e., the electromechanical and the electronic. This invention relates to electronically controlled phase shifters and in the preferred embodiments to the reflecion type of electronically conrolled phase shifter. A reflection type phase shifter employs some waveguide configuration (e.g., the various waveguide hybrids) to separate the phase shifter reflected wave from the incident wave and operates on the principle that, if a waveguide is terminated in a pure reactance, all energy incident on the termination will be reflected, and the phase shift of the reflected Wave will be a function of the magnitude of the reactance. One known device of this type employs a semiconductor diode shunted across the waveguide and positioned approximately a quarter-guide wavelength from the short circuit termination, see US. Patent No. 3,235,820, issued to J. Munushian et al., Feb. 15, 1966, and assigned to the same assignee as is the present invention. To produce a large phase shift with such a device requires high Q diodes. Typical characteristics of a good, solid state varactor are a Q of the order of when operated at X-band frequencies, and a microwave power handling capability of the order of less than 1 watt. Phase shifters employing semiconductor diodes as the variable reactance control element are sensitive to changes in ambient temperature, incur losses typically of 0.5 db or more, are limited in power handling capacity to less than 1 watt, and are susceptible to permanent damage or change in characteristics due to RF power overload.

It is therefore a primary object of the present invention to provide an improved electronically controllable phase shifter which overcomes various disadvantages which are inherent in previous phase shifters.

It is another object of the present invention to provide a phase shifter employing a unique plasma variable reactance device as the variable reactance electronic control element.

It is another object of the present invention to provide a reflection type, microwave, plasma variable reactance device phase shifter which is capable of handling relatively high RF power in both continuous (CW) and peak modes, has low RF loss, requires low DC driving power and low currents and voltages, is lightweight and compact, is relatively insensitive to changes in ambient temperature, is able to withstand overloads in RF power, and provides continuous hence precise control in phase shift over its designed range of phase shift.

These objects are accomplished according to the pres ent invention as follows. One preferred embodiment of the plasma variable reactance device phase shifter of the present invention employs a 3 db short slot coupler, which separates the incident waves from the reflected waves, connected to a gas-filled plasma variable reactance device subassembly which embodies the electronic control portion of the device. The subassembly comprises two sections of rectangular waveguide with the two ports at one end terminated by shorting plates and with the two ports at the opposite end sealed with low loss RFwindows. There are eight plasma variable reactance devices mounted four on each side of the subassembly in symmetry about the common broadwall. The preferred plasma variable reactance device construction includes an electron emitting cathode located on one wall of the waveguide, an anode for collecting electrons positioned on the opposite wall, and a grid electrode in the electron path between the emitter and the collector. With the eight plasma variable reactance devices of the above-described embodiment in operation, electrons are injected into each waveguide (from oppositely paired gun assemblies) and collected at the common broadwall which serves as a mutually grounded anode. The injected electrons from a given gun assembly produce a column of ionized gas (or plasma) which is localized or confined into a well-defined column by a DC axial magnetic field provided by a permanent magnet. In operation, opposite pairs of the eight plasma variable reactance devices are activated simultaneously; the plasma columns in a common transverse plane having the same plasma density and consequently the same microwave shunt reactance.

Another preferred embodiment of the invention provides for the generation of a plasma column common to two waveguide sections with a single variable reactance device gun assembly, thus reducing the total number of variable reactance devices and the total DC sustaining and driving powers by 50%. This embodiment employs four circular apertures in the common broadwall, the center of each of which is in direct alignment with the center axis of electron emission of one of the four plasma variable reactance devices. These apertures permit the electrons injected by the cathodes to pass through the common [broadwall and to the opposite walls unimpeded A single variable reactance device gun assembly thus produces a uniform plasma column common to the two waveguide sections; this is equivalent to two plasma columns, the microwave reactances in the two sections being controllable simultaneously with equal magnitudes. These sections are RF isolated by at least 20 db over 360 degrees range of phase control-an indication that negligible RF energy is conveyed from one section to the other.

One plasma column developed in full height waveguide for X-band use provides approximately degrees of reciprocal phase shift. The phase shifter of the invention thus requires eight variable reactance devices (in the first embodiment described above) and four variable reactance devices (in the second embodiment described above) to give at least a 360 degree range of phase control at its design (center) frequency (f Additional variable reactance devices can be employed to extend the range of phase shift.

The phase shift introduced by the plasma variable reactance device is a function of the reactance of the plasma column which in turn is determined by the plasma density of the discharge. Then density is preferably varied by controlling either the grid voltage with respect to the cathode, or alternatively the cathode voltage with the grid voltage maintained close to the cathode voltage. Continuous phase shift over a range of 360 degrees is obtained by operating in succession each veriable reactance device (or each variable reactance devices pair) over its maximum range or by driving all variable reactance devices (or varactor pairs) in parallel to maximum range. The phase shift versus voltage characteristic of the device will depend upon the RF frequency, the spacings that are chosen between the variable reactance devices, and the manner by which all variable reactance devices are brought to maximum plasma density. All variable reactance devices can be controlled simultaneously of programmed according to the desired phase shift characteristics.

In the preferred embodiments of the invention (relating to reflection type phase shifters) the gas-filled waveguide portion containing the plasma variable reactance device(s) is in that portion of the waveguide configuration containing standing waves, The reflection type phase shifter is thus distinguished from the transmission type (to which this invention is also applicable) wherein the phase shifting control element is positioned in a waveguide portion which contains only traveling waves.

These and other objects and advantages of the present invention will be more fully understood by reference to the following detailed description when read in conjunction with the attached drawings in which like numerals refer to like elements and in which:

FIGS. 1A and 1B are, respectively, a schematic diagram and an associated potential distribution curve showing how electron injection takes place through a grid sheath;

FIGS. 2A and 2B are, respectively, a cross-sectional view through a low discharge power triode type electron injection plasma variable reactance device and a graph showing the potential distribution curve of the device;

FIGS. 3A and 3B are, respectively, a cross-sectional view through a low noise triode type electron injection plasma variable reactance device and a graph showing the potential distribution curve of the device;

FIG. 4 is a cross-sectional, partly schematic view through a triode type electron injection plasma variable reactance device integrated with an X-band waveguide;

FIG. 5 is a cross-sectional view through a plasma variable reactance device having a tetrode configuration;

FIGS. 6A and 6B are, respectively, a schematic diagram and an associated potential distribution curve for the tetrode type electron injection plasma variable reactance device such as that of FIG. 5;

FIG. 7 is a perspective view of a plasma varactor phase shifter which illustrates a preferred embodiment of the invention;

FIG, 8 is a cross-sectional view through the plasma variable reactance device subassembly of the phase shifter of FIG. 7;

FIG. 9 is a cross-sectional view of a plasma variable reactance device subassembly which illustrates another embodiment of the invention; and

FIG. 10 is a graph showing phase shift as a function of relative discharge voltage of a series of plasma variable reactance devices developed in full height waveguide for X-band use.

Since the phase shifter of the present invention employs a unique plasma variable reactance device as the variable reactance element thereof, and since an understanding of the plasma variable reactance device is important in understanding the present invention, the following detailed description will begin with a description of the plasma variable reactance device itself.

A plasma variable reactance device is a device producing a plasma of controlled density over an appropriate volume. The plasma acts as a dielectric whose dielectric constant e is determined by the electron density n of the plasma and is given by Heald & Wharton at p. 6 in Plasma Diagnostics with Microwaves, John Wiley & Son, 1965, New York, as:

i lt mi where:

It is seen that an increase in electron density n results in a reduction of dielectric constant e, which means essentially that the plasma behaves as an inductive medium, the inductive effect of the plasma being controlled by controlling n As an example, at X-band lO c.p.s.), in order to make 9:0, it can be shown that a plasma density n =10 electrons/cm. will be required. This value can readily be obtained by means of gas discharges.

The RF losses of a plasma variable reactance device can be calculated by means of Equation 1 and are best expressed in terms of the Q of the reactance presented by the plasma to the RF fields. Q is defined as:

QzwW/P (2) where W=Kinetic RF energy stored in plasma P=RF power dissipated in plasma.

Power dissipation in the plasma is caused by electronion and electron-neutral collisions, which appear through the factor 7 in Equation 1. To evaluate Q from Equations 1 and 2, it may be assumed to the first approximation that the plasma is uniform and occupies a volume V (constant dielectric constant 6 within the volume V). Then:

where:

E=RMS RF electric field.

From the above relationship and Equation 1:

Equation 3 shows that in order to make a high Q (low RF loss) plasma variable reactance device, it is necessary to use a gas discharge in which the collision frequency v is low. In the type of discharge devices to be considered, electron-neutral collisions predominate over electron-ion collisions and therefore the problem is to minimize the electron-neutral collision frequency. To obtain this goal, it is necessary to minimize the gas pressure for a given electron density n In a conventional positive column discharge, this can only be done at the cost of a prohibitive discharge sustaining power. The practical limit for the Q of a plasma variable reactance device using a positive column discharge has been found to be of the order of 3 at X-band, which is too low to be of practical interest. The following described device utilizing the principle of electron injection according to the invention operates at a lower gas pressure for a given electron density n and for a given discharge power than positive column and other conventional discharge devices.

The gas discharge to be used in electron injection plasma variable reactance devices is characterized by the injection of electrons from an injection boundary in a plasma sheath into the gas to be ionized, at an energy higher than the ionization potential of the gas. This electron injection takes place through a grid (plasma) sheath, as can be seen from the potential distribution curve of FIG. 1B, that is locked to a mesh or constriction. The electron injection energy is approximately equal to the applied discharge voltage and can therefore be externally control-led. By adjusting the electron injection energy to a value equal to or larger than about 1.5 times the ionization potential of the gas used, the following advantages are obtainable: (1) the ionization cross section is close to its maximum value; this permits operation at relatively low gas pressures, much lower than conventional discharges; and (2) most of the energy imparted to the electrons is effectively used for ionization; this results in a high ionization efficiency. Operation at low gas pressure means low electron-netural collision frequency and high Q. High ionization efiiciency means low discharge power.

FIG. 2A shows an electron injection plasma variable reactance device incorporated into a waveguide 11. Here, the inner surface of the waveguide 11 also acts as an anode 12 and collects electrons injected by a cathode 13, whereby a plasma column 14 is formed therebetween. The cathode 13 can be supported by any convenient insulative means or by heavy lead wires such as wires 15 connected to two terminals of a terminal board 17. Another of these terminals is connected internally to the waveguide structure itself by means of a wire 16 so that a potential ap pears between the anode 12 and the cathode 13 by appropriate connection of the terminals to an adjustable DC voltage source (not shown). Also mounted Within the waveguide 11 is a partition 19 provided with an aperture 21 disposed opposite an emitting surface 23 of the cathode 13. Across this aperture is disposed a conducting grid or mesh 25 constructed of materials such as nickel, copper or molybdenum, for example. The grid 25 is DC-insulated from the waveguide 11 by means of an annular insulating member 27 fabricated from a suitable dielectric material such as, for example, ceramic. The grid 25 is connected to a terminal of the terminal board 17 by means of wire 29. As noted above, the grid 25 is insulated from the waveguide 11, but by means of the insulating member 27, it is capacitively coupled to the waveguide wall so as to prevent RF leakage out of the waveguide 11 through the aperture 21. An ionizable gas such as neon, xenon or krypton, to name but a few, is confined by conventional means such as quartz windows (not shown) at the ends of the waveguide in order to allow the propagation of microwave energy through the volume wherein the gas is maintained.

In order to reduce the discharge power required for a given plasma density n and to help localize the plasma in a well defined column, such as the column 14, a moderate axial magnetic field H, can be applied. Typically, a field between 100 and 500 Oersted is adequate for this purpose. By confining the plasma column 14, this field reduces substantially the ion loss by radial diffusion, thus reducing the power required to sustain the discharge. The confining magnetic field can be provided by a permanent magnet circuit. Because of the relatively low value of the field required, the magnetic material and the pole pieces can be incorporated in the waveguide 11, making up the walls of the waveguide 11 in the vicinity of the plasma variable reactance device.

The basic potential distribtuion of the above-described configuration is shown in FIG. 1B. As noted before, the electron acceleration and injection into the active part of the discharge takes place through a plasma sheath locked to a mesh or constriction. The higher plasma density is found in the region between the grid 25 and the anode 12. The lowest mean electron energy is found in the region between the cathode 13 and the grid 25 where the electrons have a substantially Maxwellian energy distribution at a temperature of the order of the cathode temperature. The plasma in either region can be used as the electronically controlled reactive element. For maximum discharge power, the region between the grid 25 and the anode 12 can be used. This is the configuration shown in FIG. 2. On the other hand, for minimum noise radiation, the region between the cathode 13 and the grid 25 can be used. This configuration is shown in FIG. 3, wherein, as compared with FIG. 2, like reference numerals denote like elements. The cathode 13, however, is supported by conventional insulative means (not shown) in a conventional microwave choke configuration 31 to prevent RF losses from the cathode area. The specific potential distribution curves of each of the devices shown in FIGS. 2A and 3A are shown in FIGS. 2B and 3B, respectively.

From FIG. 1, it can be seen that the electric injection plasma sheath is sufficiently thin that practically no electron-neutral collisions take place during the electron acceleration through this sheath. The gas pressure is adjusted such that the ionization mean free path for electrons at the injection energy (corresponding to the discharge voltage) is at least of the order of the distance L, which is the mean distance travelled by the electrons between the injection boundary, here located adjacent the grid, and the anode surface. Gas pressures lower than that described will result in a lower electron-neutral collision frequency and a higher Q, however, with little improvement in ionization efiiciency.

The plasma density and reactance (dielectric constant) may be conveniently controlled either by means of the grid potential, or by means of the anode potential. The grid potential controls the discharge current, reducing discharge current and plasma density as the grid is made more negative. For operation in the mode shown in FIG. 1, the grid potential is kept close to the cathode potential by appropriate connection to a potential source (not shown) and the mesh size of the grid is chosen such that the largest grid hole dimension d is smaller than the electron mean free path L. For a gas such as neon, for example, at a pressure of 0.3 torr, this means that d 0.3 cm. The anode potential controls the energy of the injected electrons and the plasma density by controlling the discharge voltage, V in the range (above the ionization potential of the gas use-d) where the ionization cross section increases substantially with increasing voltage. In fact, the larger the cross section for a given discharge current, the larger the rate of ion generation and the larger the resultant electron density n By using the configuration of FIG. 2, the following data was obtained:

Waveguide.Standard X-band size Gas.-Xenon Gas pressure-20 mtorr Discharge conditions for a plasma density such that EEO at a frequency of 9.87 kmc. were:

Discharge voltage.-E20 volts Discharge current-l5 ma.

Discharge power.0.3 watt Magnetic field.-150 Oersted RF performance was charactered by quality factor.--

FIG. 4 illustrates a preferred example of a plasma variable reactance device integrated with an X-band waveguide. Here, a waveguide 41 includes an aperture 43 about which is disposed a variable reactance device housing structure 45. Within the structure 45 is mounted a directly heated cathode 47, a grid mesh 63, a grid supporting structure 65, and feed through pins 51. These pins are vacuum sealed to the insulating material 61 such as ceramic or glass. The grid 63 is connected to ground through a resistor 42 in series with a voltage source 44. The waveguide 41 (and therefor that portion thereof which acts as the anode for the variable reactance device) is also connected to ground. A voltage source 46 supplies heater power to the cathode 47. The cathode is held at a particular potential with respect to the grid 63 and the waveguide (or anode) by means of a resistor 48 and voltage source 50 in series therewith. It is noted that the cathode can be of the indirectly heated type as well as of the directly heated type as shown in FIG. 4. In order to substantially reduce any loss of microwave energy through the aperture 43, a microwave quarter wavelength choke 53 of conventional design has been incorporated. This plasma variable reactance device is operated in the same manner as the plasma variable reactance device of FIG. 2A. In a particular device constructed according to this embodiment of the invention, the cathode consisted of an oxide coated nickel ribbon 0.002 x 0.015" x 0.75" wound into a helical shape; the grid (a mesh of 0.002 tungsten wires spaced 0.010") was supported on a partition having a 0.22" cm. I.D. aperture; and the aperture in the waveguide was 0.25" cm. in diameter.

In order to simplify the suppression of RF leakage through the aperture containing the grid 63, the triode type structures can be modified into tetrode structures as shown in FIG. 5. In this embodiment, as in any of the embodiments, a waveguide 71 is constructed from permanent magnet side walls 73 and a pole piece 75 forming the top wall (and the anode) and a pole piece '77 forming the bottom wall. Pole piece 77 contains a sealed plug assembly 79 having pins 81 insulatively mounted in insulating member 83.

As described in connection with the embodiment of FIG. 2, the use of an axial magnetic field, H as provided by pole pieces 75 and 77, helps to localize the plasma in a well defined column such as column 85 in order to reduce the required discharge power for a given plasma density n. An indirectly heated cathode 87 is shown supported by wire leads connected to two of the pins 8-1 which also supply heater current to the filament element within the cathode 87. In order to stabilize and support the cathode 87 in this position, glass or ceramic insulators 89 support an outer grid shell 91 which in turn supports the cathode 87 by means of an annular insulator 93. The outer grid shell 91 includes a control mesh or grid 95 through which electrons emitted by the cathode 87 and later injected into the plasma must pass.

Adjacent the control grill 95 on the opposite side thereof from the cathode 87 is mounted an anode grid 97 which is mounted on a conductive disc 99 connected to walls 73 and forming a conductive path therewith. The disc 99, of course, includes an aperture 101 across which the grid 97 is positioned. The portion of the disc surrounding the aperture 101 is tapered on the side adjacent the cathode 87 to facilitate the positioning of the control-grid 95 close to the anode-grid 97. The remaining pins 81 are connected to appropriate sources of potential and to the various elements of this device in the same manner as described in connection with the embodiment shown in FIG. 2.

In this tetrode structure, the anode mesh or grid that is mounted on a conductive partition is at the same potential as the waveguide and can, therefore, be in direct contact with it, thus most effectively suppressing RF leakage through the aperture in the partition irrespectively of the location of the aperture in the waveguide. The potential distribution for the tetrode configuration is shown in FIG. 6. The spacing between the two grids is not critical but is preferably smaller than the spacing between the anode-grid and the anode. The grid mesh openings of the anode-grid are best chosen to be equal to the size of the openings of the control grid, or coarser if RF leakage does not become excessive with larger openings. To maintain good discharge efiiciency, it is advantageous to have the wires of both grids approximately registered. The control of the reactance of a tetrode device of the type of FIG. is similar to that of a triode device, the first control grid of the tetrode device having the same function as the single grid of the triode device.

It is noted that the plasma variable reactance devices described above can be enclosed in a sealed-off container that is transparent to the electromagnetic energy into which the variable reactance is to be introduced. This has the advantages of having the gas restricted to the volume within the sealed-off container thereby obviating the necessity of providing a waveguide structure that is sealed to prevent the escape of gas. The gas container material can be ceramic, for example, which has a low dielectric characteristic.

Furthermore, it should be noted that the magnetic field shown for the purposes of plasma confinement in FIG. 5, for example, can be adjusted to a value such that the electron cyclotron frequency corresponds approximately to the frequency of the RF wave or fields to be affected by the plasma variable reactance device. Taking advantage of the cyclotron resonance permits further reductions in discharge power, gas pressure, and RF losses, and leads to a further increase in variable reactance device Q. The magnetic field required in this configuration, however, will be greater than that required for confinement of the plasma column only.

From the foregoing, it will be evident that the plasma variable reactance device has a relatively high Q, a very low R'F insertion loss, a high RF power handling capability, and the plasma is maintained substantially free from unwanted oscillations.

Having described the plasma variable reactance device itself in some detail, the phase shifter of the present invention will now be described.

FIG. 7 shows a plasma variable reactance device phase shifter 202 designed for operation at X-band frequencies. The phase shifter 202 includes a plasma variable reactance device subassembly 204, a 3 db short-slot coupler 206, and an adapter 208.

The plasma variable reactance device subassembly 204 includes two rectangular waveguides (see FIG. 8) with the two ports at the closed end 210 terminated by shorting plates and with the two ports at the opposite (or open) end 212 sealed with low loss RF windows (similar to those shown in FIG. 9). The purpose of the windows is to contain the gas (preferably xenon) which fills the subassembly 204. Eight plasma variable reactance devices 214 (214A-214H) of triode design are mounted four on each side of the subassembly 204 in symmetry about a common broadwall (see FIG. 8).

The short-slot coupler 206, which is connected to the open end of the subassembly 204, separates the incident waves from the reflected waves. Such couplers are well known, form no part of the present invention, and need not, therefore, be described here in great detail. The adapter 208 comprises a waveguide 216 for incoming (or outgoing) RF waves, and a waveguide 218 for outgoing (or incoming) RF waves.

FIG. 8 is a cross-sectional view through the subassembly 204 of FIG. 7 and shows the two waveguides 220 and 222 having a common broadwall 224. Two of the eight plasma variable reactance devices 214, i.e., 214A and 214B, are shown. The plasma columns 226 and 228 are produced by the injection of electrons into the waveguides 220 and 222, respectively. The columns 226 and 228 are confined by a DC magnetic field produced by permanent magnets 230 and 232, and magnetic pole pieces 234A and 234B, 236A and 236B, 238 and 240. The magnetic circuit could, however, be constructed external to the waveguide walls but such that a magnetic field is oriented parallel to and along the column axis. The plasma variable reactance devices 214 are of the type shown in FIG. 4. The electron injection energy is approximately equal to the applied discharge voltage and can, therefore, be externally controlled. By adjusting the electron injection energy to a value 1.5 to 2 times the ionization potential of the gas in the waveguides 220 and 222, the probability of the impact ionization of gas atoms reaches close to its maximum value. It is also possible in the electron injection mode of discharge to produce the plasma at the required densities in the waveguide with a relatively low gaS pressure, much lower than in a positive column of glow discharges. This leads to the significant improvement of the plasma quality factor Q and of the RF power handling capability of the plasma and hence of the device.

When the variable reactance devices 214 are in operation, electrons are injected into the waveguide from oppositely paired variable reactance device gun assemblies (e.g., 214A and 214B) and are collected at the common broadwall 224 which serves as a mutually grounded anode. The variable reactance device pairs are operated simultaneously so that plasma columns in a common transverse plane, produced say, from variable reactance devices 214A and 214B, have the same plasma density and consequently, the same microwave shunt reactance.

FIG. 9 is a cross-sectional view through another plasma variable reactance device subassembly 260 (useful, e.g., in the phase shifter of FIG. 7) of the present invention. The subassembly 260 is designed such that each plasma column is equivalent to two plasma columns of the design of FIG. 8, thus reducing the total number of plasma variable reactance devices (required to produce 360 degrees of phase shift) and the total DC sustaining and driving powers by 50%. FIG. 9 shows four circular apertures 262, 264, 266 and 268 in a common broadwall 270 for the two rectangular waveguides 272 and 274. The center of each aperture is in direct alignment with the center axis of electron emission of one of the four variable reactance devices 276, 278, 280 and 282. The apertures 262, 264, 26-6 and 268 permit the electrons injected by the variable reactance device cathodes to pass through the broadwall 270 unimpeded to the opposite wall. By this means a single variable reactance device gun assembly produces a uniform plasma column common to the two waveguides 272 and 274, Whose microwave reactances in the two waveguides 272 and 274 are controlled simultaneously with equal magnitudes. Both theory and experiment show that the waveguides 272 and 274 are RF isolated by at least 20 db over 360 degrees range of phase control, an indication that each plasma column conveys negligible RF energy from one waveguide to the other and that each behaves as two independent plasma reactances although it is in fact operated as a single reactance. The disposition of two guns to each side also results in balancing out differences in plasma density that might arise on account of finite electron and ion trapping at the interface.

FIG. 10 is a graph showing the amount of phase shift produced by a series of four plasma columns developed in full height waveguide (partially filled with xenon) for X-band use, as a function of the relative discharge voltage of the variable reactance devices. As described above, in one mode of operation the first plasma column (e.g., the column nearest to the shorting plates) is produced (by the use of one variable reactance device as in FIG. 9 or by the use of two variable reactance devices as in FIG. 7) then the second, the third, and the fourth in that order. The amount of phase shift produced is generally not a linear function of the relative discharge voltage. In the embodiment which produced the results shown in FIG. 10, the relative discharge voltage varied from to 20 volts, and most of the phase shift occurred in the 1620 volt range.

Additional embodiments employing other configurations of plasma variable reactance devices and waveguides will be evident to one skilled in the art. For example, in a waveguide structure as shown in FIG. 8, the gun assembly could be mounted in the common broadwall between the two waveguide sections such that a single cathode with a grid on each side thereof would produce two separate plasma columns, one in each waveguide section. Further configurations using coaxial waveguides will also be evident. Although in all of the above-described embodiments the plasma column is produced in a transverse plane of the waveguide it is clear that the invention is not limited thereto but the column can be produced so as to extend, for example, obliquely down the waveguide from one side to the other.

In the reflection type phase shifter described above the variable reactance devices can be positioned at any point in the waveguide containing the standing waves with the exception of the nodes. The plasma column would be ineffectual at the node positions. The optimum positions for a series of variable reactance devices used to produce four plasma columns in each of two waveguides such as is shown in each of FIGS. 7 and 9 is as follows. The distance between the waveguide termination or short and the first variable reactance device is such as to produce a maximum phase shift in the RF signal by the variable reactance device near the center of the frequency range in which the phase shifter is designed to operate. Another way of stating this is that the distance between the waveguide termination or short and the first variable reactance device is such as to tune the total susceptance of the variable reactance device and the section of the waveguide therebehind (between the variable reactance device and the termination) to resonate at the center of the susceptance range of the variable reactance device at the center of the frequency range in which the phase shifter is designed to operate. It has been found that the distance between each of the following or succeeding variable reactance devices, for optimum efliciency, should be the same. This distance, for example, between the first variable reactance device and the second variable reactance device is adjusted so that the total susceptance of the second variable reactance device and the first variable reactance device-wall combination therebehind produces a maximum phase shift at the center of the frequency range in which the phase shifter is designed to operate.

Using the variable reactance device phase shifter configuration as shown in FIG. 9 in combination with a 3-db slot couple, the following data was obtained:

Frequency.--X-band Bandwidth. l2%

Phase shift (continuous).--0360 Excess noise temperature.- l00 K.

RF average power handling capabilities.- 10O w.

Operating conditions were:

Discharge voltage.- 20 volts Discharge current per variable reactance device. 20

Filament power per variable reactance device.- O.6 w.

Xenon gas pressure.a20 mtorr As mentioned above, although the preferred embodiment of the present invention relates to a reflection type phase shifter, the present invention is also applicable to transmission type phase shifters in which one or more plasma variable reactance devices are placed in series to provide the required phase shift. In a transmission phase shifter of this type one variable reactance device is not able to produce as much phase shift as in the preferred reflection mode. For example, one variable reactance device may produce aphase shift of about 30 degrees as compared to a phase shift of over degrees in the preferred embodiment. Thus a larger number of variable reactance devices would be required to produce 360 degrees of phase shift resulting in larger losses, larger size, higher weight, etc.

What is claimed is:

1. A phase shifter of the reflection type for electrically controlling the phase shift in radio frequency electromagnetic energy propagated through a waveguide comprising:

a first waveguide section having one end terminated by shorting plates;

a second waveguide section connected to said first section adapted to separate incident waves from reflected Waves;

means for maintaining a gaseous medium in at least a portion of said first section;

a plasma variable reactance device mounted in said first section for producing a well-defined volume of plasma, which plasma presents a lumped reactance to said energy so that said energy undergoes a phase shift which is a function of the reactance of the plasma and which reactance is in turn a function of the plasma density;

means for electrically varying the density of said plasma whereby the phase shift is electrically controllable;

said plasma variable reactance device comprising an electron emitting cathode, an anode for collecting electrons, a grid positioned between said cathode and said anode, and means for controlling the potential of said cathode, said anode and said grid with respect to each other including means for continuously varying the potential therebetween to continuously vary plasma density.

2. The apparatus according to claim 1 including:

magnetic field producing means for localizing said plasma in a well-defined column between said cathode and said anode.

3. An electrically controllable microwave phase shifter for presenting an electrically controllable variable reactance to radio frequency electromagnetic energy propagating through a waveguide comprising:

a waveguide section;

means for maintaining a gaseous medium in said waveguide section;

means mounted in said waveguide section for producing a plasma column therein, and controlling the density thereof, which plasma column presents a reactance to said energy, said means comprising:

an electron emitting cathode,

an anode for collecting electrons,

a grid positioned between said cathode and said anode, and

means for maintaining the anode voltage with respect to the cathode voltage and the grid voltage with respect to the cathode voltage so that the grid voltage is maintained close to the cathode voltage, said last named means including means for continuously varying the voltage between said anode and said grid to continuously vary plasma density.

4. The apparatus according to claim 3 including magnetic field producing means for localizing said plasma in a column between said anode and said cathode.

5. An electrically controllable microwave phase shifter of the reflection type for presenting an electrically controllable variable reactance to radio frequency electromagnetic energy propagating through a waveguide comprising:

a first waveguide section comprising two adjacent rectangular waveguides having a common wall therebetween, said waveguides having the two ports at one end thereof terminated by shorting plates;

a second waveguide section connected to the opposite end of said first section and adapted to propagate said energy thereto and therefrom and adapted to separate incident waves from reflected waves;

means substantially transparent to the propagation of said energy for maintaining a gaseous medium in said first section;

at least one plasma variable reactance device mounted in said first section for producing a plasma column therein, which plasma column presents a lumped reactance to said energy, said plasma variable reactance device comprising:

an electron emitting cathode;

an anode for collecting electrons, which anode comprises said common broadwall;

a grid positioned between said cathode and said anode;

and

means for continuously varying the relative potential of said cathode, anode and grid to continuously control and vary the density of the plasma in the plasma column.

6. The apparatus according to claim 5 in which:

said controlling means comprises means for controlling at least one of the grid voltages with respect to the cathode voltage and the cathode voltage with the grid voltage maintained close to the cathode voltage.

7. The apparatus according to claim 6 including magnetic field producing means for localizing each of said plasma in well-defined columns between respective ones of said cathode and said anode.

8. An electrically controllable microwave phase shifter of the reflection type for presenting an electrically controllable variable reactance to radio frequency electromagnetic energy propagating through a waveguide comprising:

a first waveguide section comprising two adjacent rectangular waveguides having a common wall therebetween, said waveguides having the two ports at one end thereof terminated by shorting plates;

a second waveguide section connected to the opposite end of first section and adapted to propagate said energy thereto and therefrom and adapted to separate incident waves from reflected waves:

means substantially transparent to the propagation of said energy from maintaining a gaseous medium in said first and second sections;

said common wall being provided with four circular apertures and including two plasma variable reactance devices mounted at each of said two rectangular waveguides, the center axis of electron emission of each one of said plasma variable reactance devices coinciding, respectively, with the center of one of said four apertures whereby each of said plasma variable reactance devices produces a plasma column to each of said rectangular waveguides, each plasma column presenting a lumped reactance to said energy.

9. The apparatus according to claim 8 in which said plasma varactors comprise:

an electron emitting cathode;

an anode for collecting electrons, which anode comprises said common broadwall;

a grid positioned between said cathode and said anode;

and

means for maintaining each of said cathode, anode and grid at a predetermined potential.

10. The apparatus according to claim 9 in which said controlling means comprises means for controlling at least one of the grid voltages with respect to the cathode voltage and the cathode voltage with the grid voltage maintained close to the cathode voltage.

11. The apparatus according to claim 10 including magnetic field producing means for localizing each of said plasmas in a well-defined column between respective ones of said cathode and said anode.

12. An apparatus for producing and electrically controlling the amount of phase shift in radio frequency electromagnetic energy traveling through a waveguide, said device comprising:

shorting plates in said waveguide to terminate said waveguide to produce standing waves in said waveguide;

means for producing a plurality of plasma columns in said waveguide, including electric control means for controlling the density of said plasma so that the reactance thereof is controlled and the amount of phase shift can be controlled, and means for magnetically localizing said plasma in a well defined column, there being a suflicient number of said plasma columns to provide at least 360 of phase shift;

the plasma column closest to said waveguide termination being positioned at such a distance so as to produce the maximum phase shift at the center of the frequency range in which the phase shifter is designed to operate; and

said plasma columns subsequent to said closest plasma column being spaced from each other so as to produce the maximum phase shift at the center of the frequency range in which the phase shifter is designed to operate.

13. An electrically controllable microwave phase shifter of the reflection type for presenting an electrically controllable variable reactance to radio frequency electromagnetic energy propagating through a Waveguide comprising:

a first Waveguide section comprising two adjacent rectangular waveguides having a common wall therebetween, said waveguides having the two ports at one end thereof terminated by shorting plates;

a second Waveguide section connected to the opposite end of first section and adapted to propagate said energy thereto and therefrom and adapted to separate incident waves from reflected waves;

means substantially transparent to the propagation 0f said energy from maintaining a gaseous medium in said first and second sections;

said common wall being provided with a plurality of apertures and including a plasma variable reactance device so that the center axis of electron emission of each plasma variable reactance device coincides with the center of one of said apertures whereby each of said plasma variable reactance devices produces a plasma column to each of said rectangular waveguides, each plasma column presenting a lumped reactance to said energy.

References Cited UNITED STATES PATENTS 2,813,999 11/1957 Foin 315-39 2,817,045 12/ 1957 Goldstein et a1. 315- 39 2,837,693 6/1958 Norton 315-39 3,235,768 2/ 1966 Magnuski.

3,235,820 2/ 1966 Munushian 33331 HERMAN KARL SAALBACH, Primary Examiner. LOUIS ALLAHUT, Assistant Examiner.

US. Cl. X.-R. 

